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THE term idiopathic hyperCKemia
(IH) is used to define a persistently elevated serum concentration of creatine kinase (CK) without weakness or other significant neuromuscular symptoms.1 Asymptomatic IH is more common in males and may occur at any age. A mild transient increase in serum concentrations of CK may be caused by muscle injury and exercise and may occur in normal people. Persistently abnormal levels of CK may be the only apparent feature of endocrine or metabolic myopathies, limb-girdle muscular dystrophy, and other hereditary or acquired myopathies, particularly in their early stages.1 Weglinski et al.2 studied a series of subjects with IH and found positive contracture test results in 49%, but no correlation emerged between CK levels, histologic changes, and malignant hyperthermia (MH) susceptibility. In a recent review of individuals with IH,3 the authors found a high percentage of autosomal dominant cases and no specific findings. In most cases, pathogenesis is unknown and clinical management is unclear. Though clinically asymptomatic, IH patients could be susceptible to MH. Here, we report a series of 37 Italian subjects with IH, in which biopsy studies including the in vitro
contracture test (IVCT) were performed.

Materials and Methods

According to Italian regulation regarding observational and retrospective studies, a simple notification of the hospital ethics committee is sufficient. We have regularly notified the hospital ethics committee of the plan of the current study.

In this retrospective study, we included subjects who came to our observation in the past 4 yr with elevated serum concentrations of CK discovered incidentally (reference range, 70–170 UI/l). The CK test was performed because of minimal neuromuscular symptoms (cramps, myalgia, and others), in approximately half the subjects; in the others, the CK test was part of routine serum chemistry. We examined 37 patients (26 males and 11 females ranging in age from 11 to 69 yr). Persistent hyperCKemia was determined by four monthly measurements at rest (no exercise for 1 week) before performing muscle biopsy. Maximum and average concentrations of CK are reported in table 1. Neurologic evaluation was performed by manual muscle testing (British Medical Research Council Motor Grading Scale). No subject had a history of anesthesiologic complications or a family history of neuromuscular disorders or anesthesiologic incidents. Occasional causes of hyperCKemia, such as malignancies, drug and alcohol abuse, thyroid and parathyroid disorders, infections, and hematologic diseases, were excluded. None of the subjects were using statins or other drugs potentially capable of inducing hyperCKemia. All subjects underwent routine serum chemistry, including serum myoglobin. Clinical and laboratory investigation of various family members did not reveal any familial cases of IH. To discover subclinical myopathies, electromyography of the bilateral deltoid, biceps brachii, anterior tibial, and rectus femoris muscles was routinely performed in all cases. Open muscle biopsy from the quadriceps femoris was performed in all subjects. The specimens were examined with routine stains (hematoxylin and eosin; modified Gomori trichrome; cytochrome c
oxidase; succinate dehydrogenase; NADH dehydrogenase; adenosine triphosphatase at pH 10.4, 4.6, and 4.3; periodic acid–Schiff; and Sudan) and reactions for myoadenylate deaminase, myophosphorylase, and phosphofructokinase. Immunocytochemical analysis was performed using antibodies against dystrophin (Dys1, 2, and 3); α, β, δ, and γ sarcoglycans; caveolin; dysferlin; emerin; and merosin. Muscle specimens were also processed by standard methods for ultrastructural examination.

Table 1. Summary of the Results of the Clinical, Laboratory, and Muscle Biopsy Studies for Each Individual

In vitro
contracture testing was performed according to the caffeine–halothane contracture protocol of the European Malignant Hyperpyrexia Group on fresh specimens of vastus lateralis muscle.4 Briefly, patients were considered MH susceptible (MHS) if a contracture of at least 2 mN occurred after exposure to 2 mm caffeine and 2% halothane, MH equivocal if muscle specimens contracted in response to only one of the triggering agents, and MH negative if contracture response was normal to both drugs.

Mutation screening of RYR1 gene was performed with denaturing high-performance liquid chromatography (WAVE 3500 HT system; Transgenomic, Omaha, NE) and sequencing analysis (ABI PRISM 310 genetic analyzer; Applied Biosystems [Applera], Foster City, CA) of the three regions of the gene in which mutations result to be clustered (exons 2–17, 39–46, and 90–104).

Muscle biopsy analysis is regarded as the principal tool for screening asymptomatic subjects with IH. Our initial screening allowed us to make a precise diagnosis in 3 (8.1%) of 37 patients. The percentage of final diagnoses has varied in the few series reported so far.5–7 In a recent study on “clinically normal subjects” with chronic hyperCKemia,5 a diagnosis was achieved in 55% of cases. However, the authors described severe biopsy findings but not clinical details and included subjects with serum CK levels greater than 500 U/l, which may partly explain the difference. Differences between the various series of subjects are presumably due to different inclusion criteria. It is important to define the concept of “asymptomatic subject.” For example, whereas occasional myalgias or cramps may be found in normal people, persistent myalgias or cramps, mostly after exercise, may be significant symptoms even in the absence of local weakness. Extensive biochemical studies are advisable in cases with positive biopsy findings.

At the time of evaluation, the remaining 34 patients fulfilling diagnostic criteria for IH were clinically stable and did not report muscle weakness. Histopathologic analysis of muscle biopsies revealed nonspecific myopathic lesions. The most frequent alterations were variation in fiber size, followed by nuclear internalization and minor mitochondrial changes, or a variable combination of these changes. We found no relationship among biopsy findings, serum levels of CK, and age of patients. Even minor symptoms, observed in some patients, were not related to serum levels of CK or biopsy changes. The treatment of patients with persistently elevated serum levels of CK, usually without clinical symptoms, is a difficult and puzzling problem for anesthetists, neurologists, and physicians.2,8,9 In the current study, only three subjects had completely normal muscle biopsies; clinical management of subjects with IH should therefore include a clinical and laboratory follow-up because apparently normal patients may develop neuromuscular disorders in the future. IH probably includes different undiscovered neuromuscular disorders. Our study suggests that with current diagnostics, muscle biopsy does not reach an etiologic diagnosis in most subjects with persistently increased serum levels of CK but without significant myopathic symptoms. Although increased serum levels of CK are a valid reason to screen for underlying neuromuscular disorders, it is debatable to consider this finding predictive of MH susceptibility, unless the subject belongs to a family with a history of anesthesiologic complications.8,10–12 Subjects with IH may be part of the clinical spectrum of MH, but subclinical myopathies may give false-positive IVCT results. No certain correlation has been reported between serum levels of CK and MH susceptibility.7,8,10–13

A remarkable finding of the current study was the rarity of MH susceptibility among subjects with elevated serum levels of CK; of 37 patients undergoing ICVT, only 1 (2.7%) was MHS. The percentage of MH susceptibility among individuals with IH is not clear; reports in the literature show variable percentages.2,9,14 However, in large cohorts of patients with a high percentage of subjects positive to ICVT, patient selection was not reported. Sometimes they were enrolled on the basis of anesthesiologic complications in the family. To date, there is no agreement about what, if any, histopathologic findings are characteristic of MH susceptibility. In a series of 83 cases selected on the basis of anesthesiologic incidents, myopathic findings such as variation in fiber size, nuclear centralization, and fiber necrosis were found in MHS subjects, but no changes were detected in MH-negative subjects. In other studies, no histomorphologic differences were found between MHS and MH-negative patients. In the current study, the MHS and MH-equivocal subjects did not show any peculiar histopathologic findings with respect to other patients with IH.2,7 Ultrastructural abnormalities involving the T-tubule system, such as unusual swelling and vesiculation, have been reported. Our findings are in accord with these reports. The equivocal response of a subject with central core changes may be due to variation in concentration of halothane and caffeine during the procedure, especially if the equivocal response regards halothane. Furthermore, it is possible that not all muscles of a single subject react in the same way to trigger agents.

The relatively low percentage of MH susceptibility among patients with IH may therefore not be a surprising finding. Some authors suggest that CK levels can only be used to screen for MH susceptibility if associated with anesthesiologic complications in the family.9 Patients with susceptibility to MH and normal CK serum concentrations of CK have been reported.2 We suggest that muscle biopsy study and IVCT are useful for clinical treatment of subjects with IH; MHS subjects or those with subclinical myopathy should avoid trigger anesthetics, whereas subjects with IH should only avoid depolarizing muscle relaxants such as succinylcholine. No subject with IH should be treated with statins or other drugs that may trigger rhabdomyolysis. The patients of the current series undergo clinical evaluation and assay of serum CK every 6 months. None have yet shown muscle weakness or other additional symptoms; no further diagnosis has been reached, and CK values continue to be elevated but below their maximum values.

We suggest that muscle biopsy is still a useful, though not very sensitive, diagnostic tool in IH, because it enables potentially treatable disorders, such as inflammatory myopathies, to be discovered. MH susceptibility seems to be low among subjects with IH; serum concentrations of CK should not be regarded as a screening index for MH. No morphologic finding typical of IH or MH susceptibility was identified by muscle biopsy.